Missing css style makes Googlebot render a site falsely with heavy consequences for ranking

Hi, I had submitted this same bug on ampproject/amphtml (#16538) but was advised by some John Pettitt to file the bug again here – so I repeat it here. Thanks for helping!

**Starts here:**
I had made a new AMP site for a customer. It did not rank well after a while with Google. So I looked into it. In Google Search Console (GSC) there is a feature "fetch as Google" where you can see how Googlebot renders (!!!!!!!) a site compared to a browser. Googlebot there rendered the pages after the navigation bar completely white and wthout content.
This obviously was punished by Google as cloaking.
I went into it and found that the ampstart component "Navigation – Responsive Menubar" did not have a css height in units. This caused only the Googlebot (not the browsers) to cover the page content with the menubar's white background color – thus hiding the content and in letting assume Googlebot an attempted cloaking.
See screenshots from GSC while I was working on the bug and after having fixed it.



**How do we reproduce the issue?**

Run the fetch-as-Google-feature in Google Search Console on a standard responsive menubar from ampstart.

**What browsers are affected?**

No browsers – but only Googlebot when rendering (!!!!) a page.

**Which AMP version is affected?**

Powered by AMP ⚡ HTML – Version 1530567757224 https://www.physiomuenchen.de/

Start with the standard responsive menubar from ampstart and reproduce the issue using Google Search Console's fetch-as-Google feature. Compare Googlebot's rendering with a browser, then verify that the menubar no longer hides the page content and that the rendered result is correct.

Written by the indexing model from the issue text.